Strain softening and strain localisation in irreversible deformation of snow

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Abstract

The aim of this work was to visualise heterogeneous deformation in snow under controlled
laboratory conditions. Heterogeneous deformation was observed for both homogenous and
heterogeneous loading conditions. Understanding deformation of snow is important in many
scientific fields including vehicle traction, avalanche forecasting, and winter sports.
This thesis investigates the deformation behaviour of snow on the centimetre scale under
moderate strain rates (0.005 to 0.1 s-1) when subject to one-dimensional compression or to
indentation. In order to allow controlled and repeatable snow deformation experiments, a
new type of artificial snow was developed. This snow type was examined by low
temperature scanning electron microscopy and by traditional avalanche observer’s
methodology. Penetrometer experiments were conducted on the artificial snow and on
natural seasonal snow in Scotland. The two snow types were found to be similar: results
obtained on artificial snow are thus applicable to natural snow. A reproducible technique of
manufacture and a thorough characterisation of the artificial snow are presented.
One-dimensional compression experiments were conducted on the artificial snow. The
experiments were in confined compression in a specially constructed apparatus, designed to
provide for back-lit photography. Images were taken at 0.25 second intervals and analysed
using digital image correlation, thus providing 2D strain fields. With careful control of
photographic parameters, it is demonstrated that process of applying tracer substances to the
snow is not necessary, thus allowing an unprecedented resolution.
Spontaneously-forming strain localisations were observed for the first time, indicating strain
softening behaviour. Damage was observed to propagate through the specimen as a moving
front, resembling a wave. The force required to propagate the front remained nearly constant
until the whole specimen was compacted, at which point a new front formed and the process
repeated.
The experimental method was extended to 2D indention experiments with a range of sizes
and shapes of indenter. Complex deformation fields were observed, extending up to 6 times
the width of the indenter on each side. Observed deformation included tensile tearing as
well as compression and shear. The maximum local strain achieved in the indentation
experiments was similar to that achieved by the first compaction front in one-dimensional
compression.
The work here presented has implications for snow deformation generally: strain localisation
introduces a characteristic length, which may prevent scaling of models or results. The
indentation results are particularly relevant to snow penetrometry, where indentation
experiments are used to try and extract microstructural information from buried snow layers
for the purpose of avalanche prediction. The common assumption that the penetrometer
interacts only with snow very close to its tip may need to be reconsidered.